Abstract: By metallographic microscope, electron microscope etc. analyzing, it is obtained that surface serious decarburization of the C70S6 steel surface seriously causes the connecting rod expansion-broken bits dropping out, and internal quality defects. By optimizing the content of Al, V, and P, the ferrite constituent in steel decreases from orignial 5. 5% 〜8. 6% to 2. 7% 〜4. 8% ,and by using measures including continuous casting superheat reduced from≤35 ℃. to 30 ℃，casting speed reduced from 0. 80 ~0. 90 m/min to 0. 65 ~ 0. 75 m/min, electromagnetic stirring current at the end increased from 200 A to 300 A； coating the cast billet before rolling, air-fuel ratio in the high temperature section of the heating furnace reduced from 0. 7 ~ 0. 8 to 0. 5 ~ 0. 6, heating temperature increased to 1 200 ~ 1 250 °C ； and increasing cooling rate after rolling, the segregation of C in the center of the C70S6 steel is reduced to 0. 99 ~ 1. 04, the depth of the surface decarburization layer is ≤ 0.50% d, the grain size rating is 5. 5 ~ 6. 0, and the ferrite content is ≤5.0% , successfully solved the problem of bits dropping in batches of connecting rods.